Synchronous Optical Network (SONET)/Synchronous Digital Hierarchy (SDH) protocol is widely used for physical-layer data transport. SDH is the European equivalent of the SONET standard. A reference in this document to SONET is therefore intended to refer to SDH as well as SONET.
As is well known in the art, it is highly desirable to successfully complete every request for a connection through a telecommunications network. A connection that cannot be completed successfully, for example because a mapping of the connection through a switch cannot be found, is said to be “blocked”. It is also well known that Application Specific Integrated Circuits (ASICs) can be interconnected in a Clos network pattern to provide a versatile switching node (or shelf). In principle, such a Clos network may be designed to be unconditionally non-blocking by providing sufficient hardware resources. For example, a three-stage Clos-type network can be made non-blocking by providing a sufficiently large number of nodes in the center stage. In particular, a three-stage Clos network supporting P input connections into the ingress stage, and M intra-switch connections through the center stage, can be made completely non-blocking by providing that M≧2P−1. However, as the desired number of input connections increases, the number of intra-switch connections (M) through the center stage required for non-blocking performance (and the associated cost) becomes prohibitive. This imposes an economic limitation on the bandwidth capacity of a shelf.
However, as traffic volumes through communications networks increase, the bandwidth capacity of any one shelf can easily become exhausted, resulting in undesirable blocked connections. One way of addressing this problem is to increase the size of each shelf. However, this solution requires that a network service provider absorb the cost of replacing a small shelf with a new, larger capacity shelf. In view of the substantial cost of each shelf, many network service providers are reluctant to adopt this option.
An alternative solution is to augment a small shelf with one or more additional shelves (which may be of equal or greater size), to obtain the required total capacity. For example, consider a communications network in which the aggregate demand for bandwidth is four times the capacity of a shelf. An unconditionally non-blocking cross-connect can be achieved using a 4×4 matrix of shelves. A more efficient architecture can be obtained by interconnecting shelves into a CLOS network. This later approach enables an unconditionally non-blocking cross-connect using a 4×3 matrix of shelves.
A significant limitation of the above-noted techniques is the number of shelves required to obtain a non-blocking cross-connect. In particular, if the aggregate demand for bandwidth is N times the bandwidth capacity of a shelf, then a “brute force” approach requires N×N shelves, while a CLOS network requires 3×N shelves. The large number of shelves increases the size, complexity and cost of the cross-connect.
It is known to inverse multiplex traffic into two or more lower rate signals which are then transported over respective channels, and then subsequently recombined. For example, U.S. Pat. No. 6,002,692 (Wills) teaches a system in which a Synchronous Optical Cross-connect (SONET) signal (e.g., an OC-48c at a 2.488 GHz line rate) is inverse multiplexed into multiple Asynchronous Transfer Mode (ATM) cells which are then transported across a network through respective ports at a lower rate (e.g., 622 MHz). In cases where data of a single SONET frame is carried within two or more ATM cells, each of the involved cells is provided with a respective sequence number so that the cells can be placed into the correct sequence for reassembly of the original SONET frame.
The system of Wills is typical of packet-based inverse-multiplexing methods, in that it requires a significant amount of processing to separate the SONET frame into ATM cell payload; formulate ATM cell headers with assigned sequencing numbers; and then re-sequence the ATM cells prior to reassembly of the SONET frame. Such systems are not easily implemented at multiple gigabits per second line rates. Furthermore, such packet-based methods are not relevant to concatenated SONET signals, where the lower-rate signals are themselves SONET signals.
An alternative approach is to inverse multiplex a high bandwidth signal into multiple substreams, which can be transported, in parallel, at a lower line rate. U.S. Pat. No. 5,710,650 (Dugan) teaches a system in which a high data rate OC-192 signal (at a 9.953 GHz line rate) is inverse multiplexed into four lower rate OC-48 substreams (at a 2.488 GHz line rate) which are transported through respective parallel channels (wavelengths). The lower line rate within each channel provides increased dispersion tolerance, so that longer fiber spans can be used without regeneration of the signals. At a downstream network node, misalignment between the OC-48 substreams (due to the differing propagation speeds of the four wavelengths) is resolved by processing each of the OC-48 substreams in parallel to extract their respective 48 STS-1 frames (each having a 51.840 MHz line rate). These STS-1 frames are then individually buffered and processed, in parallel, to eliminate any misalignment.
A limitation of the above-noted prior art systems is that, in cases where a high bandwidth signal is split between two or more substreams, successful recovery of the original high bandwidth signal traffic requires precise alignment of the payload data being transported through each channel. Maintenance of such precise payload alignment generally requires synchronized pointer processing of each of the channels. However, synchronized pointer processing between shelves is typically not supported. This is due to various factors, including control signal (e.g. master clock signals, stuff indications etc.) propagation delays and variable phase jitter resulting from differing propagation path lengths. As a result, in order to obtain the necessary synchronization, all of the substreams must still be mapped through a single shelf, which at least partially negates much of the benefit of using multiple shelves, and increases the risk of blocked connections.
A further limitation is that, in practice, the signal traffic being mapped through the cross-connect will tend to be an arbitrary mixture of high and low bandwidth signals. In this respect, a “low bandwidth signal” may be considered to be a signal that is smaller than the slicing interval used to inverse-multiplex the traffic, while a “high bandwidth signal” is larger. Thus inverse multiplexing will necessarily split a high bandwidth signal across two or more lower rate substreams. In either case, there is no assurance that any of these signals will be of a size that is convenient, from the point of view of the inverse multiplexing operation. In the case of low bandwidth signals, this can result in allocation of cross-connect resources that remain unused (because the low bandwidth signal does not occupy the entire capacity of a substream). For high bandwidth signals, a more difficult problem arises, in that the signal must be split across two sub-streams, which must then be recombined to recover the original high bandwidth signal.
Accordingly, a highly scalable system capable of efficiently mapping an arbitrary mixture of high and low bandwidth signal traffic through a cross-connect remains highly desirable.